Hitherto, the main current of flame retarders has been bromine-containing flame retarders, which are organic halide flame retarders, and antimony trioxide, which is an auxiliary (or assisting agent) therefor. However, after the occurrence of a problem that brominated dioxin is generated when the bromine-containing flame retarders are incinerated, awareness against environmental problems has been enhanced. Under such a situation, by regulations in individual countries in the world, the motion of de-halogenation has been increasing. About antimony trioxide also, the motion of the use of a substitution therefor has been strengthening since the compound is suspected of having carcinogenicity. In our country also, hexabromocyclodecane (HBCD), which is a bromine-containing flame retarder, and antimony trioxide, which is an auxiliary therefor, are controlled as specific chemical substances by a law about examination and regulation of manufacture, etc. of chemical substances (Law Concerning the Examination and Regulation of Manufacture, etc. of Chemical Substances). In connection with these substances, at the time of discharging or shifting a chemical substance having a concentration not less than a regulated concentration, or a waste obtained after the chemical substance is used, the amount thereof is obligated to be reported in accordance with the rank about harmfulness of the substance or waste.
For these reasons, in place of bromine-containing flame retarders, halogen-free flame retarders have been developed. Examples thereof include phosphorous-containing compounds such as phosphoric acid esters and ammonium phosphate, and hydrated metal compounds such as aluminum hydroxide and magnesium hydroxide (see Patent Document 1). However, these halogen-free flame retarders are poorer in flame retarder effect than the bromine-containing flame retarders. Thus, in order to gain a sufficient flame retarder effect, the halogen-free flame retarders are each required to be added in a large amount. For this reason, the vulcanizate (concerned) is unfavorably lowered in mechanical strength. Thus, the halogen-free flame retarders have been desired to be improved.